What is Good AFM?
When you are looking to purchase an AFM (atomic force microscope), you need to carefully evaluate it as a whole. With AFM, there are several features which could be important depending on the application of the final system. Before you make your purchase, you need to ask yourself the following questions:
Once you have confidently answered all the above questions, and have a list of viable systems compiled, the user should make a fundamental comparison of the units that are going to best meet their individual requirements. To facilitate this, here is a comprehensive overview of what makes a good AFM.
AFMs have been proven to be reliable instruments for determining relative sample dimensions, but they have struggled to provide accurate dimensions of surface features. As the dimensions that are being found in research and industrial applications become smaller and smaller, it is now critical for AFMs to accurately and repeatedly measure the absolute dimensions of surface features.
For taking nanoscale measurements, the accuracy and reliability of measurements and the flexibility of the available options and modes are just as important as resolution. AFM performance can be influenced by the following six attributes.
An AFM tip might experience some undesired motion which can be due to the thermal expansion or contraction of mechanical parts within the instrument. This undesired motion is called thermal drift, and it needs to be minimised to accurately image samples that are smaller than 1 μm. The thermal drift rates in the X and Y direction are determined through the marking of locations of characteristic structures on the sample surface and measuring deviations from these points after conducting several scans. A good AFM will have a drift rate that is smaller than 1.5 nm/minute/oC.
This is an important factor when looking to get high-quality images with reliability and consistency. When the tip becomes blunt after coming into contact with the sample, it limits the resolution of the AFM and it will reduce the quality of the image. For softer samples, tip-surface contact not only damages the tip but the sample as well; compounding the inaccuracies of sample height measurements. The ability to preserve tip integrity will enable the user to produce consistent high-resolution and accurate data.
A good way in which to test the tip life of an AFM system is by imaging CrN, which has sharp and pointy features which are on a hard surface. If a tip becomes damaged or blunt, it is no longer able to reach the bottom of the features, and the images taken will become blurred. By choosing to invest in a good AFM, the sharpness of the top can be preserved even after 100 image scans of a CrN sample, while maintaining the same sample surface roughness.
The AFM scanner is the most important element of any AFM, as its performance determines the accuracy of the imaging results. It is crucial to determine if there are any scanner artefacts, such as non-linear and high-order background motions, to avoid distorting the resulting AFM image.
When looking for an AFM, you need to look for a scanner design which will minimise out-of-plane motion when imaging a flat surface. A good AFM scanner must be able to keep the Z out-of-plane motion within a few nanometers over the entire scan range, independent of scan size, scan rate and scanner offset.
Even the smallest and seemingly insignificant environmental vibrations can add noise to AFM results, which makes it very challenging to image small details and to characterise the flattest surfaces. To measure baseline noise or noise floor, the user needs to bring the cantilever to the sample surface and will obtain the system response to a “zero scan”. A good AFM should be isolated from vibrations to achieve a noise floor below 0.5A.
Today’s AFMs can characterise a wide range of physical properties depending on the specific sample and application needs of the customers. Of particular importance are the SPM modes which probe the following properties:
When purchasing your AFM, you need to pay careful attention to which SPM modes it will be able to support.
A good quality AFM must be able to provide a wide range of option compatibility to facilitate data acquisition in a wide range of conditions and sample environments. Of particular importance are: